When sensing, for instance, particles or gases in a fluid, such as a fluid in a ventilation duct, it is important for the sensing to be reliable. As a rule, such sensing is done by part of the fluid being made to pass a detector, such as a smoke detector, which is arranged outside the ventilation duct.
Different types of devices for such sensing are known, in which sensing occurs by fluid being diverted from the ventilation duct and being passed through a space past a detector in the same for sensing, after which the fluid is returned to the duct. If, for instance, a smoke detector senses that there is flue gas in the fluid from the ventilation duct, the smoke detector emits, for instance, a signal which can be connected to an alarm, or which can constitute an initiation signal to close the duct system etc.
These prior-art devices for sensing a fluid frequently have a not quite satisfactory function as regards safe and reliable registration of small amounts of, for instance, flue gas in said fluid. Therefore there is a need to improve the accuracy of such devices to detect the presence of gases and particles in small amounts in a fluid in, for instance, a ventilation duct.